Cure catalysts are routinely used in isocyanate cured hydroxy terminated polymer such as hyroxy terminated polybutadiene (HTPB), which is used extensively as a binder for solid rocket propellants. Cure acceleration by means of catalysis has been especially important for curing agents such as isophorone diisocyanate (IPDI), which uncatalyzed, may require several weeks at elevated temperature to react completely. Catalysis can also provide another advantage by reducing side reactions by causing the hydroxyl isocyanate reaction to be favored.
One of the most popular cure catalysts for many years has been triphenyl bismuthine (TPB). This catalyst is activated by many acids, and it was believed necessary to add some form of acid to the liquid propellant to trigger the activity of TPB. Some acids caused the reaction to be triggered so rapidly that some in the industry began to use maleic anhydride (MAN) which they believed would be converted to maleic acid (MA) at some time after addition to the propellant mix and would then activate the TPB. Magnesium oxide (MgO) was also added in the mix cycle to scavenge acid that might initiate TPB activity prematurely. An indepth investigation of the TPB/MA cure catalyst system has revealed that this hypothesis appears to be in error.
In retrospect one skilled in the art acknowledges that obtaining and controlling adequate pot life of solids filled isocyanate cure polymer systems such as polybutadienes, polyesters, etc. has been a formidable problem especially with fast reacting isocyanates or in the presence of such catalytic materials as iron oxide, chromium oxide, catocene, ferric fluoride, etc. The cure reaction consists of the chemical reaction of a di (or poly) isocyanate with the di or poly hydroxyl containing polymer to produce a filled rubber like material. Of particular concern to the skilled artisan is the pot life of a curable system.
Pot life is defined as the time the filled polymer system remains fluid enough to be processed and cast into the appropriate vessel (mold, rocket motor, etc.). This may vary widely according to the system involved. Typically, a minimum of six hours is necessary in production of most rocket motor systems. Extremely catalytic materials such as those mentioned above may reduce pot life to less than one hours. Also some of the more reactive isocyanate curing agents such as tolyene diisocyanate (TDI), may inherently produce a shorter than desired pot life.
Historically, problems in attempting to obtain longer pot life have been addressed by using lower mix temperatures, selective use of certain bonding agents, phosphine oxides, tetracycline, or less reactive isocyanates such as isophorone diisocyanate (IPDI). All of these approaches can create problems and in many instances are still inadequate.
An object of this invention is to obviate the problems associated with catalysis reaction of curing agents by employing other materials that direct the catalysis reaction in the desired direction.